1. Field of the Invention
The present invention relates to a process for preparing sugar nucleotides, which are important substrates in the synthesis of oligosaccharides.
2. Description of the Related Art
Recent remarkable progress in sugar-chain science has clarified some of sugar's physiological roles, which makes it possible to develop pharmaceuticals and functional materials based on oligosaccharides possessing physiological activities. However, only limited types of oligosaccharides are currently available on the market, and in addition, they are extremely expensive. Moreover, these oligosaccharides can be produced only on a reagent level, and a mass-production method for them has not yet been fully established.
Conventionally, oligosaccharides have been produced by way of extraction from natural substances, chemical synthesis, enzymatic synthesis, or a combination of these. Among these processes, enzymatic synthesis has been considered best suited for mass-production for the following reasons: (1) enzymatic synthesis does not require intricate procedures, such as protection and removal of protection, which are required for chemical synthesis; (2) substrate specificities of enzymes enable the synthesis of oligosaccharides having highly structural specificities. In addition, recent progress in recombinant DNA technology have made it possible to mass-produce various types of enzymes economically and in large quantities, also contributing to establishing the superiority of enzymatic synthesis over other processes.
Two processes for the synthesis of oligosaccharides by use of enzymatic synthesis are available: a process that makes use of the reverse reaction of a hydrolase, and a process that makes use of a glycosyltransferase. The former has an advantage that it can employ inexpensive monosaccharides as the substrate, but, because it employs the reverse reaction to the hydrolysis reaction, it is not necessarily the best process for the synthesis of oligosaccharides in terms of yield and application to oligosaccharides possessing a complicated structure.
In contrast, the latter makes use of a glycosyltransferase and has an advantage over the former in terms of the yield and application to the synthesis of oligosaccharides possessing a complicated structure. Moreover, the mass-production of various types of glycosyltransferase enabled by recent progress in recombinant DNA technology also contributes to realization of this process.
However, sugar nucleotides, which are sugar donors used in a synthesis that makes use of a glycosyltransferase, are with few exceptions still expensive, and are provided only in small amounts on reagent levels. For example, there have been reported processes for preparing uridine diphosphate-N-glucose (UDPG), which is a donor of glucose contained in core parts of a variety of physiologically active sugar-chains, and the processes include a chemical synthesis method making use of uridylic acid (UMP) and glucose 1-phosphate (G-1-P), and a yeast cell method making use of UMP and glucose as the substrates (T. Tochikura et al., J. Ferment. Technol., 46, 957 (1968), S. Shirota, et al., Agric. Biol. Chem., 35, 325 (1971), and S. Watanabe and I Takeda, and Agric. Biol. Chem., 36, 2265 (1972)), but problems still remain to be solved before industrial production is realized.
The inventors of the present invention have carried out studies on a process for preparing UDPG by conventional yeast cell methods, in which only a small amount of UDPG was produced and 60% or more of the added UMP was converted to uridine triphosphate (UTP) or uridine diphosphate (UDP). Therefore, these methods have no value as a process for the synthesis of UDPG in the actual production.
Accordingly, the present invention is directed to providing a process for preparing sugar nucleotides such as UDPG by improving conventional yeast cell methods.